1. Field of the Invention
This invention generally relates to a material dispensing system and, more particularly, to a computerized system which places a strip or bead of material, having a substantially constant cross-sectional area upon a desired object, or is capable of selectively placing a bead if a variable cross-sectional area is required upon such an object.
2. Discussion
Dispensing systems are normally used to place several beads or strips of material, each comprising an adhesive or sealant, upon an object such as an automobile windshield or door frame assembly. It is very desirable to ensure that these material beads each have a specific and constant cross-sectional area, in order to allow the beads to cooperatively perform their desired function, while preventing material build-up, which may cause the material to flow to undesired portions of the object. This undesirable and improper material flow usually requires the object to be cleaned, thereby causing a waste of much time and resources, and may even cause the object to adhere to undesired portions of an assembly, resulting in undesirable assembly operation.
Prior dispensing systems usually include a source of material (i.e. a supply pump and metering unit), which provides a constant flow of material. The material is then directed to an applicator which is adapted to be moved in close proximity to and around the object. In order for these prior dispensing systems to apply a material bead or strip, having a constant cross-sectional area (or several beads, each having substantially similar cross-sectional areas), the applicator must be moved at a constant velocity, proportional to the rate of flow of the material emanating from the dispenser. Moreover, if a bead having a variable cross-sectional area is required, the applicator must be moved at a varying speed during bead formation.
While these prior dispensing systems have worked well for some objects, they failed to provide beads, each having a desired constant (or selectively variable) cross-sectional areas upon objects having very sharp or rounded corners or edges. This was due to the fact that the applicator is usually required to be stopped or reduced in velocity, around these sharp corners or edges. This reduction in applicator velocity causes some of the beads, or even portions of a single bead, to have irregular and undesirable cross-sectional areas, since the material flow rate is not concomitantly reduced.
One approach, used to alleviate these problems, involved stopping both the applicator and the flow of material entering the applicator, as soon as the applicator reached a very sharp corner or edge. While this approach is somewhat useful, it requires extensive software programming of the dispensing system and often results in an excessive build-up of the dispensed material in the corner or rounded edge portion of the object, or in a failure to provide needed material over some of the object portions.
Another approach, to alleviate problems associated with these prior dispensing systems, required the changing of the dispensed material flow rate to correspond to the slower applicator velocity, needed to traverse the sharp corner or edge portions of the object. This approach was found to be unsatisfactory since a change in material dispensing flow rate does not instantaneously result in a change of material flow rate at the applicator. This flow rate delay is due to the fact that a certain pressure drop is normally established between the material dispenser and the dispensing applicator, which are normally many feet apart.
When the flow rate of the dispensed material, at the dispenser, is quickly reduced to a new flow rate, the new flow rate is not observed at the applicator, until a new pressure drop is established between the material dispenser and the applicator (commonly called lead/lag). This delay is known to be caused by the distance between the tip and the dispensing unit; the viscosity of the utilized material, the compressibility of the material; the elasticity of the conduits connecting the dispenser with the applicator; and the magnitude of the rate of change in the flow rate, at the metering or dispensing unit.
Moreover, this flow rate gradient is unacceptable, since it also causes accessible build-up at the sharp corner or edge portions of the object or a failure to provide needed material over some of the object portions.